Abstract
Abstract The Lesser Himalayan Arun Tectonic Window comprises the Upper Precambrian, polymetamorphosed metasediments of the Nawakot Complex. These are tectonically overlain by Precambrian gneisses of the Higher Himalayan Kathmandu Complex. The main phase metamorphism within the Higher Himalayan rocks predated thrusting along the Main Central Thrust. By contrast, the main metamorphism (M2) within the Nawakot Complex commenced synchronously with it being underthrust during D2 along the Main Central Thrust Zone beneath the amphibolite facies rocks of the Kathmandu Complex. During D2 rocks on both sides of the Main Central Thrust Zone underwent intensive mylonitization within a several hundred meter wide zone of non-coaxial shearing. Although the stratigraphic order within the Nawakot Complex is the right way up, rocks within the Arun Tectonic Window show a concentric inverted metamorphic zoning, ranging from lower greenschist facies in the core of the window, up to lower amphibolite facies at the outer edge of the window. This ‘inverted metamorphism’ (M2) of the Nawakot Complex originated as a result of syn- to post-D2 downward heat conduction from the ‘hot’ hanging wall of the Main Central Thrust onto the ‘cold’ footwall. This heat flow also caused the strong retrograde overprinting at the base of the Kathmandu Complex. After a static period, a late- to post-M2 NNE-shortening (D3) commenced along the Main Boundary Thrust. The Arun Tectonic Window is the result of doming of the Main Central Thrust during D4 followed by erosional exhumation.
Published Version
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